Conventionally, a rare earth magnet such as Nd—Fe—B type has been used in a room temperature environment, for example, in a voice coil motor (VCM) of a hard disk drive or in a magnetic resonance imaging (MRI) device, and therefore, heat resistance has almost never been required so far.
In recent years, this type of rare earth magnet is expanding its application, for example, to an EPS motor of general vehicles, a driving motor of hybrid electric vehicles (HEV), or a motor for FA (robot or machine tool). Along with such expansion of the application range, the rare earth magnet is required to have heat resistance and be capable of withstanding use in a relatively high temperature environment. This tendency is strong particularly in the application to automobiles.
The most common method for elevating the heat resistance of the rare earth magnet is to increase the coercive force, and a method of adding Dy, Tb or the like at the melting of Nd—Fe—B-based alloy has long been employed.
Recently, an attempt to increase the coercive force by diffusing a Dy metal into the inside from the surface of the rare earth magnet has been made. For example, International Publication No. WO2006/064848, pamphlet (claims, FIG. 1, etc.) discloses an Nd—Fe—B-based sintered magnet and a production process thereof, where a fluoride, oxide or chloride of Dy is treated by reduction to cause diffusion and penetration of a Dy metal into a grain boundary phase from the surface of the Nd—Fe—B-based sintered magnet, whereby the grain boundary is modified to give a high Dy concentration on the magnet surface and a low Dy concentration in the inside of the magnet.
Also, for example, JP-A-2004-304038 discloses a rare earth sintered magnet and a production process thereof, where a Dy or Tb metal film is formed by sputtering on the surface of the rare earth sintered magnet, followed by subjected to a heat treatment, thereby thermally diffusing Dy or the like inside of the magnet.
In addition, JP-A-62-206802 describes a method of mixing a Dy—Nb alloy powder, a Dy—V alloy powder or the like with an Nd—Fe—B-based alloy powder and sintering the powder mixture to obtain a sintered magnet.
However, these conventional techniques have the following problems. That is, in the method of adding Dy, Tb or the like at the melting of an Nd—Fe—B-based alloy, the coercive force is increased utilizing a principle of increasing the magnetic anisotropy by replacing Nd of Nd2Fe14B crystal with Dy or the like, but in accordance with this principle, Dy or the like and the Fe atom are coupled together magnetically antiparallel to each other, which disadvantageously causes reduction of remanence.
The technique described in WO2006/064848 where a Dy metal is caused to diffuse and penetrate into a grain boundary phase from the surface of a rare earth sintered magnet is applicable to a sintered magnet, but this technique can be hardly applied to a magnet produced through hot molding such as hot press or hot plastic working such as hot extrusion. The reasons therefor are as follows.
According to the technique described in WO2006/064848, a heat treatment at a high temperature of around 1,000° C. is necessary for thoroughly reducing and diffusing Dy. In the case of a sintered magnet, the magnet itself is sintered at about 1,100° C. and therefore, hardly causes grain growth under the above-described heat treatment conditions, and the problem of reduction in the coercive force due to increase of the grain growth can be almost disregarded. On the other hand, the magnet produced through hot molding or hot plastic working allows grain growth under the above-described heat treatment conditions, so that the elevation of coercive force by virtue of Dy diffusion and the decrease of coercive force due to grain growth cancel each other. Also, when the grain size is increased, the magnetic domain becomes unstable and the coercive force decreases. For these reasons, it has been difficult to apply the technique described in WO2006/064848 to a magnet produced through hot molding or hot plastic working to enhance the heat resistance thereof.
As for the technique described in JP-A-2004-304038 where a metal film of Dy or Tb is formed by sputtering on the surface of a rare earth sintered magnet and such a metal is thermally diffused into the inside of the magnet, an expensive apparatus is necessary for the formation of metal film. Furthermore, because of batch production of small amounts) the productivity is low.
In both of the techniques described in WO2006/064848 and JP-A-2004-304038, Dy or the like is caused to diffuse into the inside of the magnet from the magnet surface and therefore, while the concentration of Dy or the like is high in the surface part of the magnet, the concentration of Dy or the like is low in the inside of the magnet and, as a result, the magnetic characteristics of the entire magnet are likely to become non-uniform. This is disadvantageous in obtaining high magnetic characteristics over the entire magnet. Other than WO2006/064848 and JP-A-2004-304038, a large number of methods for diffusing Dy into the inside of the magnet from the magnet surface are disclosed, but these methods all are relying on the diffusion from the magnet surface and although there are some differences, the non-uniformity of magnetic characteristics due to difference in the Dy concentration between the surface and the inside of a magnet cannot be avoided.
The method described in JP-A-62-206802 where a Dy—Nb alloy powder or the like is mixed with an Nd—Fe—B-based alloy powder and the powder mixture is sintered, the sintering temperature is as high as about 1,100° C. Accordingly, the grains have a size of from 5 to 10 μm and in view of single domain theory, this is disadvantageous in obtaining a large coercive force and is fundamentally not preferred. In addition, since the element Dy mostly diffuses into the inside of a main grain during high-temperature sintering, despite an increase of the coercive force, there is a drawback that reduction in the remanence becomes large.